1. Field of the Invention
The invention relates to the addition of color mixing optics and optical feedback to produce uniform color throughout the light beam produced by a multi-color LED illumination device.
2. Description of Related Art
Multi-color LED illumination devices (also referred to herein as light sources, luminaires or lamps) have been commercially available for many years. For example, Cree has marketed a variety of primarily indoor downlights, troffers, and other form factor luminaires that combine white and red LEDs to provide higher color rendering index (CRI) and efficacy than conventional white LEDs alone can provide.
Philips Color Kinetics has marketed many multi-color LED products, however, most are restricted to indoor and outdoor saturated wall-washing color and color changing effects. Recently, Philip's introduced the “Hue” product, which has an A19 form factor that provides colored, as well as white light. This product combines blue, red, and phosphor converted LEDs to produce saturated blue and red light, pastel green, and white light that can be controlled by a computer or smartphone. The phosphor converted LEDs produce a greenish light, but cannot produce a saturated green, like that of a red/green/blue/white (RGBW) LED combination. Since the Hue product has an A19 form factor, color mixing is achieved with simple diffusers arranged in the output light path above the LED package. Color accuracy in the Hue product is susceptible to LED aging, since it does not use optical feedback to compensate for the change in luminance over time for each of the differently colored LEDs.
Conventional color mixing optics typically use light guides, which tend to be large and inefficient. The rule of thumb for a light guide is that it should be about 10 times longer than the dimensions of the multi-color light source. A typical 90 Watt halogen bulb produces about 1200 lumens. An array of many large LEDs is necessary to produce such output light. For instance, 1200 lumen output LED arrays from Cree are about 5-6 mm in diameter. If such a light source comprised multi-colored LEDs, a 50-60 mm light guide would be needed to properly mix the colors. Considering that the light beam needs to be shaped after color mixing, the dimensions needed for a light guide become prohibitive.
No products currently exist on the market that provide both accurate white light along the black body curve and saturated colors. Further, no such products exist in a PAR form factor that provide uniform color throughout the standard 10, 25, and 40 degree beam angles. As such, a need exists for improved techniques to produce full color gamut LED light sources that do not change over time and that have uniform color throughout the entire light beam.